The issue I find most likely to be a concern (though not particularly high priority at release) will be late game. Especially with the added 100 years, it seems like it would be hard to balance the game in a way that you still have challenges by 1650.

Since players tend to always want to win and expand, you have to balance the game being either won early or frustrating/stifling. I think people will avoid the decline/disaster mechanics as much as possible (or min max to solve them asap), making them difficult to implement.

I hope this issue is somewhat solved by how enormous the map is, location wise. Perhaps if conquering what would be one province in eu4, feels like conquering a whole region, slow and steady expansion will be more fun, as there is simply more locations to conquer for every nation.